A particulate matter sensor which measures the quantity of particulate matter (PM) contained in exhaust emissions and a particulate matter sensing system using such a sensor are known (Publication of US patent application No. 2013/0256296 A1). The particulate matter sensor includes a pair of electrodes separate from each other and a heater which heats the electrodes. The particulate matter sensing system is equipped with the particulate matter sensor and a control circuit connected to the particulate matter sensor.
The control circuit is designed to switch between a measuring mode and a burning mode in a control operation. In the measuring mode, voltage is applied between the electrodes of the particulate matter sensor. This causes particulate matter emissions to be converged by electrostatic force, so that electrical current flows between the electrodes. The amount of particulate matter contained in the exhaust emissions is calculated by measuring the value of the electrical current to determine a value of resistance of the particulate matter deposited between the electrodes. When the measuring mode continues to be performed for a while, a great deal of particulate matter is accumulated between the electrodes, so that the electrical current is saturated. In this time, the measuring mode is changed to the burning mode to actuate the heater to burn off the accumulated particulate matter. This restores the particulate matter sensor.
Recent years, development has been carried out to measure the temperature of the heater accurately. Specifically, the resistance of the particulate matter usually depends upon the temperature thereof. Therefore, as long as the temperature of the heater, that is, the temperature of the particulate matter is accurately measured in the measuring mode, it is possible to correct the measured value of the resistance of the particulate matter to achieve accurate calculation of the amount of particulate matter contained in the exhaust emissions. Additionally, when the temperature of the heater is accurately measured in the burning mode, it is possible to preciously control the temperature of the heater, thereby preventing the temperature of the heater from being excessively elevated or lowered. This eliminates a risk that the heater is deteriorated due to an excessive increase in temperature thereof, or a lack in burning the particulate matter due to an excessive decrease in temperature thereof. For these reasons, it is sought to accurately measure the temperature of the heater.
In order to accurately measure the temperature of the heater, the above particulate matter system measure an electrical resistance of the heater. The temperature of the heater and the resistance bear a relation (see FIG. 7). The calculation of the temperature of the heater is, thus, achieved by measuring the resistance of the heater.